For conventional cataract surgeries, the corneal incision method and the scleral incision method are generally known. In the corneal incision method, the operator uses a single knife such as a slit knife or a clear corneal knife to form an incision wound in the cornea and then sends the knife into the anterior chamber. In the scleral incision method, the operator cuts the cornea and then performs lamellar scleral dissection using a straight knife. Subsequently, the operator extends the incision wound to the cornea using a crescent knife and, eventually, introduces a slit knife into the anterior chamber. As a new incision method having advantages of the corneal and scleral incision methods, the transconjunctival single-plane sclerocorneal incision method is now known. In the transconjunctival single-plane sclerocorneal incision method, as illustrated in FIGS. 4(a) and 4(b), for example, the operator introduces a knife into a conjunctiva 32 at a position spaced outward, which is, for example, toward the eyebrow, from a limbus 31 by approximately 0.5 mm, and then sends the knife directly into a sclera 33. The knife is then sent into a cornea 34 until it reaches an anterior chamber 35. If a knife is introduced into the conjunctiva 32 at a position close to the eyebrow, the incision wound is covered and protected by the upper eyelid after the surgery. This reduces the changes of endophthalmitis caused by infection. Until the knife reaches the stromal layer of the cornea 34, the operator moves the knife upward, which is outward, along the curve of the cornea 34. To send the knife into the anterior chamber 35, the knife is faced slightly downward (inward) and moved parallel to or slightly upward (outward) with respect to an iris 36. This makes it easier to form a substantially linear inner incision line 37a of an incision wound 37. Then, the operator cuts the conjunctiva 32 upward to the cornea 34 at the opposite ends of a wound 37b (an outer incision line) of the incision wound 37. This allows perfusion fluid to flow to the sides of the wound 37b (the outer incision line), thus preventing the perfusion fluid from flowing into the conjunctiva 32 and causing conjunctival chemosis. As a result, as viewed along the cross section of the eyeball shown in FIG. 4(a), the knife proceeds along a substantially S-shaped movement path 38 by changing its proceeding direction sequentially through a movement path section 38a extending in the direction of arrow P to enter the conjunctiva 32, a movement path section 38b extending in the direction of arrow Q to proceed from the sclera 33 into the cornea 34, and a movement path section 38c extending in the direction of arrow R to enter the anterior chamber 35. The above-described incision methods have the advantages and disadvantages listed in Table 1. Particularly, the scleral incision method and the transconjunctival single-plane sclerocorneal incision method form a substantially cranked or S-shaped path to form the incision wound 37. This allows the incision wound 37 to naturally close without being sutured, exhibiting improved self-sealing performance.
TABLE 1TransconjunctivalSingle-planeCornealsclerocornealScleralIncisionIncisionIncisionDamage toNoneMinimal (IncisionObservedConjunctivaOnly)(Incision AndDetachment)Damage toNoneMinimal (Ablation NotObservedScleraPerformed)(Ablation AndHemostasis)Damage toObservedMinimalCorneaConjunctivalNoneExtremely Low LevelRare Even InChemosisWider Range ofConjunctivalDetachmentOperationShortLongTimeTunnel LengthShortMediumLongTunnel ShapeLinearSlightly CurvedCurvedSelf-SealingNormalImproved (Cornea Covering Wound AndPerformanceBlood Functioning Glue)
Conventionally, a medical knife used in a cataract surgery according to the transconjunctival single-plane sclerocorneal incision method includes a handle extending from a proximal end of a blade plate having a distal blade portion, as in, for example, a bevel-up type slit knife or the knife shown in FIG. 1, particularly, out of the attached drawings of Patent Document 1, which will be listed below. In this knife, the extending direction of the grip portion of the handle and the extending direction of the blade plate cross each other. As illustrated in FIGS. 2 and 5, particularly, of Patent Document 1, the blade portion of the blade plate has a front-face portion and a back-face portion. The front-face portion is formed on the upper side in the thickness direction of the blade plate, which corresponds to the extending direction of the grip portion of the handle. The back-face portion is formed on the lower side in the thickness direction of the blade plate, which faces opposite to the extending direction of the grip portion of the handle. Two blade surfaces are formed on the opposite sides in the width direction of the blade plate, which crosses the thickness direction of the blade plate, at the front-face portion and the back-face portion of the blade portion. The blade surfaces are both inclined from the middle portion in the width direction of the blade plate toward the opposite outer ends in the width direction to decrease the interval in the thickness direction. A cutting edge is formed at the outer end at which the blade surfaces of the front-face portion cross the corresponding blade surfaces of the back-face portion and extends from the distal end of the blade portion to the proximal end of the blade portion.
In a typical cataract surgery using the above-described transconjunctival single-plane sclerocorneal incision method using the medical knife according to Patent Document 1 or the like, the operator introduces the knife into the anterior chamber 35 along the movement path section 38c, which extends in the direction of arrow R, as illustrated in FIG. 4(a). At this stage, when reactive force to the force acting in the proceeding direction of the knife is applied to the blade portion, the front-face portion of the blade portion and the back-face portion of the blade portion receive the upward force that presses the blade portion upward and the downward force that presses the blade portion downward in correspondence with the shape of the front-face portion and the shape of the back-face portion. The blade portion enters the anterior chamber 35 while being translated upward or downward in correspondence with equilibrium between the upward force and the downward force. If the blade portion is translated downward, the point at which the opposite cutting edges in the width direction cross each other facilitates formation of the inner incision line 37c in a projected shape, with reference to FIG. 4(c). If the blade portion is translated upward, as shown in FIG. 4(d), formation of the inner incision line 37d in a V shape is facilitated. In both cases, the inner incision lines 37c, 37d are easily displaced offset from each other, and the above-described self-sealing performance is hampered disadvantageously.